The invention relates to an electrophotographic copier having an endless photoconductive element and an endless intermediate support which moves in synchronization with the element and under pressure accepts the powder image from the element and transfers and fixes it to the paper. Such devices are well known in the art, for example, see U.S. Pat. Nos. 4,455,079; 4,439,462 and United Kingdom Patent Application No. 2 040 226 A. Each of these electrophotographic devices transfer the electrostatically formed toner image from the photoconductor to an intermediate support at a first zone to paper at a second zone. In these devices the intermediate support is heated to melt or render tacky the toner for application on the paper to receive the toner image.
One of the troublesome problems with this method of copy transfer is that the heated intermediate support causes the photoconductor to age more quickly. In United Kingdom Patent Application No. 2 040 226 A a cooling means is used to cool the intermediate support on its travel from the second zone back to the first zone. Similarly, U.S. Pat. No. 4,455,079 uses a cooling roll to achieve the same cooling of the intermediate support through heat conductive elements.
In such devices it is possible in practice to make good quality copies over a long period if it is designed to produce copies of one single size. In such a case the photoconductive element is always loaded uniformly over a strip having the same width as that size, which means that the unavoidable aging of that strip takes place gradually and uniformly.
To compensate for the influence of such aging it is well known to regulate the intensity of the image-wise exposure or the electrostatic charging of the photoconductive element in relation to the number of copies that have been produced using the element. In the known devices, the intermediate support is cooled after leaving the second zone, based on the idea that the aging of the photoconductive element is to a large extent caused by heat transfer from the intermediate support to the element in the first zone and that the aging is more serious the higher surface temperature of the intermediate support is when it reaches the first zone.
However, if a copier of the type described above is suitable for making copies of mutually differing dimensions, the photoconductive element and the intermediate support will, when copies of the one size are being produced, be loaded in a different part than when copies of another size are being produced. As a result of such an uneven load, the photoconductive element will show uneven aging on its surface. This is particularly true when, as is often the case in practice, the device is predominantly used for producing copies of a small size (e.g., A4), to a lesser extent for copies of a slightly bigger size (e.g., A3) and only for a few copies of an even bigger size (e.g., A2). The resultant unevenness in aging becomes visible on the larger copies in the form of unacceptable, differently tinted strips in the background.
This unevenness cannot be postponed or its effects compensated for by applying the cooling measures referred to in the known devices, because they are applicable to the case of uniform aging.
The cause of said unevenness is attributable to a number of factors. For instance, the surface portions of the intermediate support which come into contact with a copy sheet in the second zone transfer heat to the sheet and, therefore, upon leaving the second zone they will be cooler than surface portions which have not come into contact with the sheet. If the intermediate support, having such a temperature pattern on its surface, comes into contact with the photoconductive element in the first zone, the element will be subjected to an uneven heat load. In addition, the surface portions of the intermediate support which come into contact with copy sheets will transfer more paper dust and image powder remnants to the photoconductive element than those portions which do not make such contact. Consequently, the photoconductive element and the intermediate support become unevenly contaminated.
Partly because of the uneven contamination, the nature of the surface of the more loaded portions of the intermediate support will change slightly (e.g., they will become somewhat rougher). This means that in the first zone, the frictional load upon the photoconductive element will be different from that brought to bear by less loaded portions.
Accordingly, the known systems and devices do not attempt to reduce or eliminate the nonuniform aging of the phtoconductor which shows up on the larger copies in multi-sized copiers.